While hydrogen is considered a "clean" fuel because the only waste product it generates is water, the conventional way to produce it relies on electricity, which is usually produced through the burning of fossil fuels. Researchers at the University of California, San Diego (UCSD), have now developed a "3D branched nanowire array" that they claim could cheaply and cleanly deliver hydrogen fuel on a mass scale.

The nanowires, which are made from abundant natural materials such as silicon and zinc oxide, mimic the structure of a forest of trees, with individual vertical "trees" sprouting hundreds of nano-sized "branches." Like forests, this structure maximizes the amount of solar energy that can be captured, with the vertical structures trapping and absorbing the light, while the flat surfaces reflect it.

Using this nanotree structure, the researchers were able to maximize the amount of solar energy captured for use in producing hydrogen in a process called photoelectrochemical water-splitting. This process usually uses planar solar cells to produce hydrogen in a process similar to the electrolysis of water, but the UCSD team says their nanowire arrays produce more hydrogen fuel efficiently.

Ke Sun, a PhD student in electrical engineering who led the project, says the vertical nanotree structure also allows very small gas bubbles of hydrogen to be extracted much faster to maximize the hydrogen gas output. Additionally, the surface area for chemical reactions has been enhanced by at least 400,000 times in the nanotree structure compared to its planar counterparts.

While the team says its nanotrees provide a cheap way to produce hydrogen fuel on a mass scale, they are aiming to go further. Like other research teams, they are looking to use the nanotree structure to mimic photosynthesis in a device that not only harnesses the power of the sun to produce hydrogen fuel, but also captures CO2 from the atmosphere to reduce carbon emissions at the same time.

"We are trying to mimic what the plant does to convert sunlight to energy," said Sun. "We are hoping in the near future our 'nanotree' structure can eventually be part of an efficient device that functions like a real tree for photosynthesis."

The team is also looking at alternatives to zinc oxide which, although it absorbs the sun's ultraviolet light, has stability issues that affect the nanotree structure over time.

Electronic microscopic image of a "nanoforest," with green tint added for contrast (Imge: Wang Research Group, UC San Diego Jacobs School of Engineering)

The light trapping effect in nanowire arrays sees photons bounced between single nanowires and eventually absorbed by them (R), allowing hydrogen fuel to be produced more efficiently compared to planar counterparts where they are reflected off the surface (L) (Image: Wang Research Group, UC San Diego Jacobs School of Engineering)